Cracks in Comptroller John Liu’s union support over the straw-donor fund-raising scandal are beginning to show.

Sources with ties to the labor movement are now publicly saying Liu could be forced to resign over the federal probe that led to last week’s arrest of his campaign treasurer, Jenny Hou, who was charged with illegal fund-raising.

“His mayoral aspirations are dead in the water,” said Scott Levenson, a political/labor consultant whose clients have included the Working Families Party, the AFL-CIO, and unions representing teachers, hospital workers and building workers.

“Conventional wisdom is that it’s not likely that Comptroller Liu will serve out his term,” Levenson said.

“At this point, it’s very difficult for the comptroller, who is entrusted with the finances of the city, to continue to govern when there’s an ongoing investigation into allegations of usurping the campaign-finance law.”

Levenson said Liu is at a political dead end: Either he was a willing participant in a scheme to skirt the law, or he didn’t know — which is equally damning.

“It’s difficult for any principal to look at what was supposedly done . . . and claim ignorance,” he said.

Levenson, who has worked on more than 100 political campaigns, added, “I don’t remember a principal who was so divorced from his campaign.”

Levenson’s not the only doubter.

“Liu was done a long time ago. It’s not a question of whether he’s done. He’s got to worry about whether he’s going to jail,” said Teamsters Local 237 President Gregg Floyd, who has clashed with Liu over pension reform.

Levenson and Floyd were publicly expressing what other union officials have said privately. But those labor leaders said they did not want to “pile on” Liu, whom they consider a strong ally.

“I don’t know about his legal problems. I see them in the paper. John Liu has been the comptroller. We work well with him, and as long as he’s comptroller, we’ll continue to work well with him,” Bloomberg said.

Hou, 25, is charged with helping Liu’s wealthy backers evade the $4,950 limit on individual contributions by using straw donors to pass along their money to his campaign, according to the federal complaint filed by Manhattan US Attorney Preet Bharara and the FBI.

Meanwhile, a key Liu bundler, Xing Wu “Oliver” Pan, has been indicted by the feds on similar charges. He was nabbed in an FBI sting operation in November.